Heathland Project Report
Introduction List of Summary Points Summary
Comparison of Sampling Methods
Results obtained using different methods of sampling may often be very different. This is well illustrated by a comparison of results obtained for a few selected species in 1997, using both the belt transect and the random quadrat sampling methods.
This highlights the need to use the same sampling method when comparing data collected at different times.
Comparison of data (mean % cover) obtained in 1997, using two
different sampling methods
There are two reasons for the different results obtained using the belt transect and random quadrat methods. The first is related to the number of samples taken. The random quadrat data is based on twice the number of samples in each section compared to the belt transect. (Quadrats were sampled every 2m for the belt transect and every 1m for the random quadrats.) The greater the number of samples taken, the more reliable are the data obtained.
The optimum minimum number of samples which should be taken can be established by constructing a species area curve for the site. This is done by taking a large number of samples and then drawing a graph of the total number of species found against the number of samples taken. This is called the species area curve.
This method is really only relevant where the vegetation is homogeneous. The heathland site is obviously not homogeneous overall, with different sections having quite different vegetation in some cases. Species area curves were therefore constructed for each section (Figure below). A species area curve for the whole site was then constructed by taking mean values of all the sections (Figure J below) At the point indicated by
P (Figure J below) the curve is levelling out to a straight line. This indicates that even if more samples were taken, the number of species found would not increase substantially. This point (14 samples) represents the minimum number of samples which will be representative for the site.
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These graphs demonstrate the effect of the number of quadrats sampled on the number of species recorded. |
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At the point indicated by P (Figure J, left) the curve is levelling out to a straight line. This indicates that even if more samples were taken, the number of species found would not increase substantially. |
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Above: The effect of the number of quadrats sampled on the number of species recorded.
Belt transect data for the site is based on between 6 -16 samples depending on the width of the section sampled at the point of the transect line. The mean number of samples per section was 7. This number was adequate for certain of the sections (3,4,5 and 9 -Figures ;C,D,E,I above) but not for the other sections. For this reason random sampling was introduced in 1997. The mean number of random samples for each section was 14, which is a sufficient level of accuracy as determined by the overall species area curve (Figure J above).
The random sampling data is also a better reflection of the vegetation on the site because the samples cover the whole area of the sections and not just a narrow band across the middle, as in the belt transect. The distribution of many of the species on the site is extremely patchy, and the greater the area sampled, the more accurate the data will be. The belt transect data, while inadequate on its own, in combination with the random samples gives a good picture of the vegetation on the site and its changes over time.
For the reasons outlined above, where changes are being compared for the years 1996 - 1998, only belt transect data is used, to allow strict comparisons to be made. Where the years 1997 and 1998 only, are being compared, an average of belt transect data and random quadrat data has been calculated to give a mean value which encompasses both methods.
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